Power Electronics EEE 4153

Silicon Controlled Rectifier (SCR)

Most important type of power semiconductor device. Have the highest power handling capability. They have a rating of 5000V / 6000A with switching frequencies ranging from 1KHz to 20KHz. Is inherently a slow switching device compared to BJT or MOSFET. Used as a latching switch that can be turned on by the control terminal but cannot be turned off by the gate.

SCR

Symbol of Silicon Controlled Rectifier

Device StructureA thyristor is a four layer pn-pn semi conductor device consisting of three pn junctions. It has three terminals: an anode a cathode and a gate. Figure shows the thyristor symbol and a sectional view of the three pn junctions.Figure -Thyristor Symbol & pn Junctions

Operating Mode

When Anode is connected with +ve terminal and Cathode is connected with Ve terminal. Gate is zero J1 and J3 Forward Biased, J2 Reversed Biased Device will not contactWhen Anode is connected with -ve terminal and Cathode is connected with +Ve terminal. Gate is zero J1 and J3 Reversed Biased, J2 Forward Biased Device will not contact To turn SCR ON, three junctions need to be Forward Biased. Anode is more +ve than Cathode and a positive Gate pulse must be applied.

Two Transistor Model of SCR

Two Transistor Model of SCR

Figure 4.3 Two-Transistor Model of Thyristor

This model is used to demonstrate the regenerative or latching action due to positive feedback in the thyristor. A thyristor can be considered as two complementary transistors. One being pnp and the other npn. The twotransistor model is shown in figure 4.3 below.

The collector current IC of a transistor is related to the emitter current IE and the leakage current of the collector base junction ICBO as...........(1) The emitter current of transistor Q1 is the anode current IA of the thyristor and collector current IC1 is given by .............(2) where 1 and ICBO1 are the current gain and leakage current respectively for transistor Q1. Similarly, the collector current for transistor Q2 is IC2 where .............(3)

where 2 and ICBO2 are the current gain and leakage current respectively for transistor Q 2. Combining the two collector currents IC1 and IC2 yields

.............(4)

When a gate current IG is applied to the thyristor

.............(5) Solving for anode current IA in equation 5 yields

.............(6)

The current gain 1 varies with emitter current IE1 which is equal to IA; and 2 varies with emitter current IE2 which is equal to Ik.

If the gate current IG is increased from zero to some positive value, this will increase the anode current IA as shown by equation 6. An increase of IA which is an increase of IE1 would increase 1 as shown in figure 4.4 and also 2 . Since

The increase in values of both 1 and 2 would further increase the value of anode current IA which is a regenerative or positive feedback effect. If 1 and 2 approach unity, the denominator of equation 6 approaches zero and a large value of anode current is produced causing the thyristor to turn on as a result of the application of a small gate current.

V-I Characteristics of Thyristor

The important points on this characteristic are : Latching Current IL This is the minimum anode current required to maintain the thyristor in the onstate immediately after a thyristor has been turned on and the gate signal has been removed. If a gate current greater than the threshold gate current is applied until the anode current is greater than the latching current IL then the thyristor will be turned on or triggered.

Holding Current IHThis is the minimum anode current required to maintain the thyristor in the onstate. To turn off a thyristor, the forward anode current must be reduced below its holding current for a sufficient time for mobile charge carriers to vacate the junction. If the anode current is not maintained below IH for long enough, the thyristor will not have returned to the fully blocking state by the time the anode-to-cathode voltage rises again. It might then return to the conducting state without an externally-applied gate current.

Reverse Current IR When the cathode voltage is positive with respect to the anode, the junction J2 is forward biased but junctions J1 and J3 are reverse biased. The thyristor is said to be in the reverse blocking state and a reverse leakage current known as reverse current IR will flow through the device. Forward Breakover Voltage VBO If the forward voltage VAK is increased beyond VBO , the thyristor can be turned on. But such a turn-on could be destructive. In practice the forward voltage is maintained below VBO and the thyristor is turned on by applying a positive gate signal between gate and cathode. Once the thyristor is turned on by a gate signal and its anode current is greater than the holding current, the device continues to conduct due to positive feedback even if the gate signal is removed. This is because the thyristor is a latching device and it has been latched to the on-state.

Methods of Thyristor Turn-on

Thermal Turn-on. Light. High Voltage. Gate Current. dv/dt.

Thyristor Turn-onA thyristor is turned on by increasing the anode current. This can be accomplished in the following ways.

ThermalsIf the temperature of a thyristor is high, there will be an increase in the number of electron-hole pairs. This would increase the leakage current. This increase in leakage current causes the anode current to increase and as a result causes 1 and 2 to increase. Due to the regenerative action, the sum 1 + 2 may tend to unity and the thyristor may be turned on. This type of turn-on may cause thermal runaway and should be avoided. Light If light is allowed to strike the junction of a thyristor, the electron-hole pairs will increase and this may cause the thyristor to be turned on. This is the principle of operation of light activated Thyristors. High Voltage If the forward anode to cathode voltage VAK is increased beyond the forward breakdown voltage VBO , high enough leakage currents will flow, causing regenerative turn-on. This type of turn-on is destructive and should be avoided.

dv/dt From equation 6, if the rate or rise of the anode to cathode voltage is high, (for example, when there is a voltage spike), the charging current of the capacitive junctions may be high enough to turn on the thyristor. A high value of charging current may cause damage to the thyristor and must be avoided. Hence, thyristors must be protected against high dv/dt and must be operated within the manufacturer's dv/dt specifications. Gate Current The injection of gate current into a forward biased thyristor would turn-on the device. As the gate current is increased, the forward voltage required to turn-on the device decreases. This is shown in figure 4.6.

Figure 4.6 Effects of Gate Current on Forward Blocking Voltage

Gate Control Circuit Design

Consideration must be given to the following points when designing gate control circuits.The

gate signal should be removed after the thyristor has been turned on. A continuous gate signal will increase the power loss in the gate junction.No

gate signal should be applied when the thyristor is reversed biased. If a gate signal is applied under these conditions, the thyristor may fail due to an increased leakage current.The

width of the gate pulse must be greater than the time required for the anode current to rise to the holding current. In practice, the gate pulse width is made wider than the turn-on time of the thyristor.